Mobile communications systems are currently being implemented at an increasing rate. Such systems generally include a plurality of geographically distributed base stations that are each responsible for servicing mobile users in a particular area, known as a cell. When a mobile user wishes to establish a communication channel with another user, the mobile user transmits a radio frequency request signal to the nearest base station. The base station receives the request signal at an antenna and subsequently transfers the request to a mobile switching center (MSC) which sets up the requested connection. The base station then acts as a radio frequency link in the communication channel until the channel is terminated.
Mobile communications systems normally employ a large number of base stations to cover a given area. As described above, each of these base stations requires at least one antenna for receiving and transmitting signals to users. The antennas are normally mounted on poles that are located at an elevated point within the cell, such as on the top of a tall building or on a mountain peak, to obtain total coverage within the cell. Because a large number of antennas are required for a typical system, it is important that the antennas be relatively inexpensive to manufacture. In addition, because of the location of the mount, it is important that the antennas be compact, lightweight, and relatively easy to install on the pole. Furthermore, the antennas should provide good performance characteristics, such as low-loss and linear operation.
In communications applications, it is very important that circuitry remain substantially linear. This is especially important in systems that utilize multiple adjacent frequency channels because nonlinearities in such systems can cause interference between individual channels of the system. That is, frequency components from one or more channels can combine as a result of the nonlinearity to form intermodulation products that appear in the frequency range of another channel. As is apparent, these intermodulation products can greatly reduce system performance. Therefore, efforts should be made to minimize system nonlinearities.
To make an antenna system more compact, a multiple layer feed arrangement can be utilized. That is, circuit structures can exist on two or more vertical layers, rather than all on the same layer, thereby reducing the overall footprint of the antenna. In such a multi-layer system, a means must be provided for coupling signals between the different layers. This coupling means must provide a relatively good impedance match between transmission structures on the different layers and should be relatively low loss. In addition, the coupling means should not create undesirable transmission modes within the antenna housing (i.e., the coupling should not radiate within the housing).
To decrease the weight of an antenna, some systems utilize air loaded transmission lines within the antenna housing to, among other things, transfer radio frequency energy between an input/output connector and each radiating element. Air loaded transmission lines, in general, require some means to suspend a center conductor a predetermined distance away from one or more nearby ground structures to achieve a required characteristic impedance. Past suspension devices invariably introduce dielectric loading to the transmission line which creates undesirable mismatches and losses on the transmission line. To reduce the effects of the mismatches, past systems placed suspension devices at quarter wave intervals along the transmission line so that reflections caused by adjacent devices would cancel. Because this practice generally requires more suspension devices than are needed for supporting/suspending the center conductors, additional weight and signal loss is added to the system.
An important consideration in designing an antenna system for use in a large communication system is ease of installation. As described above, a typical mobile system can require a multitude of antennas to cover a desired service area. Installation and servicing of these antennas can be a monumental task requiring many man-hours of labor with associated labor costs. Therefore, if the complexity of the installation process is reduced, system installation and maintenance costs can be reduced. In addition, a reduction in antenna installation complexity can reduce system installation time and increase installer safety.